Some aspects of the mechanism of tRNA biosynthesis and function in E. coli will be investigated. It is desired to answer, in part, the following questions: a) are all tRNA's made via precursors? b) is secondary structure of precursor a controlling factor in tRNA biosynthesis? c) is classical hydrogen-bonded codon-anti-codon recognition permitted for the UGA codon? and d) what role do individual modified nucleotides on a particular tRNA species play in protein synthesis? Finally, an indirectly related question, does acridine mutagenesis involve correction of mismatched sequences in recombining DNA molecules?--will also be examined. The system I have recently developed to study a tRNA precursor in E. coli will be extensively utilized. Enzymatically and genetically altered precursors will be used as substrates in experiments designed to test a particular model of tRNA biosynthesis and the role of nucleotide modification. Hydroxylamine-induced ochre-suppressing and non- suppressing acridine-induced mutants will be starting material for the UGA codon and mechanism of mutagenesis problems respectively. Additional studies involving the T4 rII system are designed to aid in clarifying the latter question.